1. Field of the Invention
This invention is concerned with a process for treating an exhaust gas containing nitrogen oxides, oxygen and ammonia with a catalyst which specifically exhibits an excellent activity to decompose unreacted ammonia in the exhaust gas as well as a high catalytic activity to decompose the nitrogen oxides. Since the catalyst can decompose ammonia in the presence of nitrogen oxides and oxygen in a treatment condition, there is no problem due to unreacted ammonia, which may be contained in the treated exhaust gas in a conventional process which uses ammonia gas as a reducing gas for the nitrogen oxides.
The process according to the present invention is applicable to various treatments of exhaust gases from, such as boilers of power plants, nitric acid production plants, steel making plants, etc.
2. Description of the Prior Art
The co-pending U.S. patent application Ser. No. 531,304, filed Dec. 10, 1974, entitled "Process for Reducing Nitrogen and Novel Catalyst Composition Useful Therefor" discloses that a catalyst consisting of titanium oxide, copper oxide and molybdenum oxide is usable for reducing nitrogen oxides contained in an exhaust gas to nitrogen by catalytically reacting the nitrogen oxides with ammonia gas.
Table 5 of the co-pending application discloses a catalyst consisting of TiO.sub.2, CuO and WO.sub.3 in an atomic ratio of 8.9:1:0.1 (Molar ratio of the catalyst is 1:0.11:0.011). The NO.sub.x reduction rate in case of this catalyst is 99.7%. Although this catalyst exhibits a good activity on reduction reaction between nitrogen oxides and ammonia, it was found by the present inventors that the catalyst having the above mentioned composition showed an unsatisfactory activity on decomposition of unreacted ammonia. Ammonia gas as a reducing agent is added to an exhaust gas to be treated in excess of a stoichiometric amount with respect to nitrogen oxides, in general, in order to avoid the presence of unreacted nitrogen oxides in a treated exhaust gas. Therefore, an excess amount of ammonia gas tends to be contained in the exhaust gas. If the amount of unreacted ammonia gas is large, a new air pollution will be caused.
There have been known a variety of catalysts for removing nitrogen oxides, which include titanium base catalysts disclosed in the copending application and catalysts of CuO--Al.sub.2 O.sub.3, CuO--SiO.sub.2, CuO--Cr.sub.2 O.sub.3, Pt--Al.sub.2 O.sub.3, etc. According to the experiments conducted by the present inventors, the catalysts of CuO--Al.sub.2 O.sub.3, CuO--SiO.sub.2, CuO--Cr.sub.2 O.sub.3 etc. other than platinum group catalysts showed unsatisfactory activity on decomposition of nitrogen oxides. Further, it was also found that the platinum group catalysts were apt to lose catalytic activities due to poisoning by sulfur oxides in an exhaust gas, and that since the platinum group catalysts exhibited a strong oxidation activity on ammonia, considerable amount of nitrogen oxides is produced during treatment of the gas at elevated temperatures.